The High Temperature Reactor and the TRISO coated fuel particle (Part II)

The inner and outer pyrolytic carbon layers both shrink and creep during irradiation. A portion of the gas pressure (30 MPa for a power of 150 mW/particle) is transmitted through the IPyC layer into the SiC. This pressure continually increases as irradiation of the particle continue, thereby contributing to a tensile stress in the SiC layer. However, countering the effect of the pressure load is the shrinkage of the IPyC and OPyC during irradiation, which pulls the SiC inward. Failure of the particle is normally expected to occur if the stress in the SiC layer reaches the fracture strength of SiC. The pressure induced failure can be reduce by providing an ample margin in the thickness of the buffer layer and by changing the fuel kernel from UO2 to UCO, which would produce a decrease in the amount of CO produced (the main gas responsible for the internal pressure). Pyrolytic carbon cracking can be eliminated by reducing the anisotropy of the PyC layer.

Chemical interaction of the SiC coating layer with fission products is another possible performance limitation of the TRISO-coated fuel particle. Previous irradiation experiments indicate that fission products of palladium and lanthanides react with the SiC layer. Corrosion of the SiC layer could lead to fracture of the coating layers or provide a localised fast diffusion path, which degrades the capability of retaining fission products within the particle. The corrosion of SiC by the fission product palladium has been observed in almost all kinds of fuel compositions, and is considered as one of the key factors influencing the fuel performance. To avoid the degradation of the coating layers caused by extensive corrosion of the SiC layer, it is necessary to limit the fuel temperature, irradiation time or increase the thickness of the SiC layer. These limitations narrow the range of operation conditions of the HTRs.

Although not a failure mechanism, the migration and release of silver (produced by the decay of Uranium) is considered an important issue since it determines the plan maintainability and service requirements. Silver can migrate through intact particles and be released into the reactor coolant system, where it will deposit on cold surfaces. Even though the release mechanism is not very well understood, it has been demonstrated that silver diffuses through the grain boundaries in SiC (Fig. 1)

Fig. 1. Silver diffusion in SiC. E. Lopez-Honorato et al. Silver diffusion in silicon carbide coatings. J. Am. Ceram. Soc. 94, 3064-3071, 2011. http://onlinelibrary.wiley.com/doi/10.1111/j.1551-2916.2011.04544.x/abstract

The High Temperature Reactor and the TRISO coated fuel particle (Part I)

One of the goals in the design of a nuclear reactor is the safe enclosure of all possible radioactive material not only during normal operation but especially during any possible accident. The High Temperature Reactor (HTR) achieves this goal by placing a small amount of fissile and/or fertile material surrounded by different layers of ceramics, effectively creating a miniature fission product containment vessel. This reactor relies on the ability of these ceramic coatings to retain all key radionuclides as long as a certain maximum fuel element temperature is not reached. Additionally, the reactor is designed in a way that in case the temperature of the reactor goes up above normal conditions the speed of the neutrons will increase, effectively shutting down the chain reaction necessary for fission to occur. Furthermore, because this reactor is constructed underground, natural heat conduction will help to reduce the temperature of the core.

The design of the coated fuel particle has changed since the early stages in the 1960’s. Although several variations of coating designs have been produced, only three designs have been used in a HTR. These three designs in chronological order are: (1) laminar, (2) BISO and (3) TRISO.

The laminar design was used briefly at the start of the HTR project. It contains a single layer of pyrolytic carbon (PyC). This design was used in the Peach Bottom, AVR and Dragon reactors. This design was abandoned after it was observed that fission product recoil damage produced early failure and release of fission products.

The BISO particle contains two types of material, a porous carbon coating (low density pyrolytic carbon, PyC), and a high density PyC coating. The porous carbon provides void volume to limit the fission gas pressure and protects the outer layer from recoil damage. The second dense isotropic coating layer is the containment to retain the fission products. The acronym BISO was derived from the fact that the two isotropic coating materials were used. BISO fuel particles are no longer part of any country’s HTR fuel design. The diffusional release of certain metallic fission products from BISO particles, particularly caesium, strontium, and silver, occurring at elevated temperatures, together with a high uranium contamination of matrix material during manufacture, led to only TRISO (Tistructural Isotropic) particles being used in all HTR modern designs.

The TRISO (Tristructural Isotropic) coated fuel particle was created in the UK as part of the DRAGON project. It consists of a microspherical fuel uranium kernel and coating layers of porous pyrolytic carbon (PyC) called the buffer, inner dense PyC (IPyC), silicon carbide (SiC) and an outer dense PyC (OPyC) (Fig. 1). The porous buffer layer provides free volume for gaseous fission products without causing excessive pressure build-ups, and isolates the structural coatings from the mechanical interactions caused by kernel swelling that accompanies fission product accumulation. It also protects the structural coatings by stopping fission fragment recoil atoms ejected from the fuel kernel. The inner dense PyC layer (IPyC) protects the SiC layer by stopping many fission products (i.e. rare earth elements) that might otherwise chemically attack it. It also prevents reaction between the UO2 kernel and chlorine containing materials released during the deposition of SiC. IPyC undertakes part of the internal pressure produced by CO2, CO and gaseous fission products. The silicon carbide layer (SiC) enhances the mechanical stability of the pressure vessel to retain gaseous fission products, and is the major containment barrier for fission products. The outer PyC (OPyC) layer gives the particle a higher temperature capability by preventing the vaporization of the SiC layer. It also protects the SiC from mechanical damage during fuel manufacture and is an additional barrier for gaseous fission products in case of disruption of the SiC layer.

Figure 1. TRISO coated fuel particle.

 

 

Transmutation of long-lived waste in fast reactors and accelerator driven systems (ADS)

 The management of the high-level radioactive waste (HLW) generated by the current fleet of light water reactors (LWR) is one of the most important issues that needs to be addressed. Due to the long-lasting radiotoxicity of the HLW this material needs to be isolated for thousands of years if directly disposed into a deep geological repository. Although minor actinides (MA)(neptunium, americium and curium) constitute a small fraction of the HLW, they are largely responsible for the long-lasting radiological toxicity and heat produced by the waste (Figure 1). For example, the heat generated by 241Am strongly influences the size of the repository to dispose HLW. Therefore, the destruction of these transuranic elements into stable or shorter-lived isotopes (transmutation) by further irradiation is of great benefit as it can reduce the long-term radiotoxicity of the waste and extend the repository capacity.

Time evolution of the radiotoxicity of high-level waste (HLW).

Under neutron irradiation MA can undergo neutron capture or fission reaction depending on the energy of the neutrons. Neutron capture is generally avoided as only produces heavier actinides, whereas fission reactions produce fission products with shorter half-lifes and lower radiotoxicity. In LWR, with neutron energies lower than 0.1 eV, MA mainly undergo neutron capture. In contrast, in Fast Reactors fission reactions are promoted by a factor of 2-6 depending of the isotope. Despite this improvement, MA fission rates can be between 15-30%, meaning that the fuel needs to be recycled a few times to ensure high performance levels. However, due to the impact of MA in the reactor core safety their concentration needs to be limited to 5% in an homogeneous cycle.

One dedicated system for the transmutation of MA is the accelerator driven system (ADS). Contrary to the fuel of the Fast Reactor, the fuel of the ADS can contain 30 wt% MA, 20 wt% Pu and 50 wt% ZrN as dilution material. It has been suggested that an 800 MWth ADS is capable of transmuting 500 kg of MA in 600 effective full power days, equivalent to the MA produced in 10 units of LWR with 1GWe.

Challenges for future Nuclear Reactor Systems (Part II)

High Temperature Reactors

Commercial reactors generally operate with an outlet temperature of around 200-300°C with the heat produced only used to generate electricity. By contrast, considerably higher temperatures (800-1000°C) can be achieved in a new generation of nuclear reactors known as High Temperature Reactors (HTR). The high temperature reactor is a particularly intriguing reactor system with exceptional safety characteristics combined with high efficiency for conversion of heat to electricity. Furthermore, direct use of the heat produced, via a heat exchanger, are envisaged also for industrial processes. For example, above 400°C the heat generated can be coupled for iron manufacturing and petrochemical processes such as the de-sulfurisation of heavy oil, petroleum refining and production of ethylene and styrene (Fig. 2). One very important product that can be obtained using the heat from a HTR is hydrogen. This element, considered essential for the future transport industry and currently used to produce liquid fuels, ammonia, methanol and other products, is presently obtained from a process called steam methane reforming. In this method, steam and methane are combined to produce hydrogen and CO2, using heat from natural gas. The heat produced by the HTR not only could replace the use of natural gas but also can be used to directly split water to produce H2 and O2 without the necessity to convert large amounts of carbon to CO2. The use of this technology could further reduce green house emissions by replacing the large amount of fossil fuels used today for industrial processes.

Figure 2. Nuclear process heat for the industry. http://www.nextgenerationnuclearplant.com/

 The safety of a HTR originates from the characteristics of its fuel and the design of the core. The fuel of the HTR is made of a very small fuel particles (0.5 mm in diameter) covered by four thin layers of ceramic making it no larger than 1 mm (Fig 3). Thousands of this particles, known as TRISO fuel, are then mixed with graphite to make the fuel elements in spherical (pebble bed design) or cylindrical (prismatic core) form depending on the design of the core. One of the main advantages of this type of fuel is that the thin ceramic coatings (aprox. 0.03 mm in thickness each) serve as a miniature fission product containment vessel designed to trap all the fission products. Under off-normal conditions, the fuel in a reactor with a power of less than 200 MW rises to less than 1600°C, and the integrity of the ceramic components is maintained.

Figure 3. The components of the pebble fuel in the High Temperature Reactor. http://www.pbmr.com

Super Critical Light Water Reactors

The supercritical light water reactor (SCLWR) is perhaps more appropriately referred to as the high performance light water reactor (HPLWR). Like the light water reactor (LWR), its coolant is water, but it is maintained under conditions where water becomes a so called super critical fluid. Thus, its characteristics change dramatically. The main advantage of this concept lies in the higher efficiency, with which the heat can be converted to electricity. With a 50% higher efficiency than a conventional system, a major impact on the system economy can be achieved. Furthermore, this leads to a concomitant decrease in the waste generated.

Molten Salt Reactors (MSR)

The final reactor selected is cooled by a molten salt. The salts are mixtures are based on lithium and sodium fluorides (LiF and NaF), which depending on their composition melt at around 700°C. The fuel is also in the form of a fluoride and is dissolved in the molten salt. Unlike all the other reactor systems, the fuel is not static. It is pumped through the reactor core, and is “cleaned” of its impurities (i.e. fission products) in an online treatment facility. This reactor exhibits great flexibility in its operation, and furthermore can be readily fuelled in a sustainable manner by thorium, an element even more predominant than uranium in the earths crust. Challenges lie in the fuel chemistry, separation technologies, and in the qualification of materials resistant to the corrosive molten salts.

The path forward

Current reactor technology will continue to play an important role. The majority of the reactors operating today belong to Generation II light water reactors (LWR). Newest LWR reactors (Generation III) have adopted additional safety technologies.

Safety of the reactor operation is essential no matter what reactors are deployed. The quest for perfection is unremitting. It can cover core design and layout. The behaviour of the fuel with its inventory of fission products during normal and off normal reactor operating conditions, during interim storage (cooling) and reprocessing, and indeed within a long term repository must be known and quantified.

Licensing by national authorities, often known as technical safety organisations (TSOs), is essential. The light water reactor fleets have generated a large quantity of operational experience and data, enabling the establishment of engineering based computer simulation codes, with sufficient predictive capability for licensing. Establishing this competence for future reactor systems will require a similar engineering type effort. Science never stands still. However, theory is developing fast, as is computational power. Though still a fair way off, it can be expected that a paradigm shift for licensing will occur. Improved computational power and modelling will lead to advanced designs, and improved less time consuming and costly paths to best possible fuels.

The recent events at Fukushima will reinforce nuclear industry’s commitment to safety. The unimaginable must be imagined, and measures taken to contain consequences of a plant failure, as successfully achieved at Three Mile Island in 1979.

 

Challenges for Future Reactor Systems (part 1)

by Eddie Lopez Honorato and Joseph Somers

The ever increasing worldwide demand for energy along with concerns over green house gas emission and future security of energy source of supply are at the forefront of energy policy across the globe. The role of nuclear energy with its low carbon imprint has rapidly become recognised, and has been promoted by well known figures that once opposed nuclear energy as Patrick Moore (co-founder of GREENPEACE). Recent events in Japan, with three nuclear reactors stricken as a result of an unprecedented natural disaster, have renewed the public debate about nuclear energy. It is not, however, the purpose of this article to contribute to this debate, rather to provide an overview of how future systems could evolve, if and when required.

Towards the end of the last century, a number of nations gathered to deliberate on the future of nuclear energy for the coming century. Thus the Generation IV International Forum, also known as GIF, was founded. Various panels were established to select the most promising reactor systems for the future, based on the following criteria:

• Safety of the reactor system

• Resource sustainability i.e. maximising the amount of energy that can be extracted from natural resources, while not compromising future generations

• Waste minimisation e.g. capability to curtail long term waste legacy

• Economy (capital, operation and decommissioning)

• Proliferation resistance, i.e. capability to guard against misuse of materials

After much debate, six reactor systems were recognised, as capable of fulfilling these criteria and are discussed below.

Fast Neutron Reactors

Nuclear reactors can be classified based on the energy of the neutrons that produces the fission reaction responsible for the heat generated in the reactor. Today’s commercial reactors use “thermal” neutrons that are slowed down by a moderator, i.e. water. In contrast, “fast” reactors use neutrons without the use of a moderator. The fast neutron reactors with sodium lead, and gas coolants (denominated SFR, LFR and GFR) optimise uranium resource use by a factor of 50 above today’s light water reactors (LWR). They pave the way for improved radioactive waste management through fuel cycle closure with recovery of valuable fissile material for irradiation in the next reactor cycles, thus improving the uranium resource utilisation. In addition, the minor actinides (i.e. neptunium, americium, curium), the major contributors to long term radiotoxicity, can be recovered and transform to elements with lower radiotoxicity (transmutation) through further irradiation. Thus, in theory at least, the long term radiotoxicity of the spent fuel can be reduced from some 100,000 years down to 300 years. Even if the transmutation efficiency is not 100%, a reduction to 1000 years seems reasonable (see figure 1), and is within the bounds one can foresee for engineering barriers against release to the biosphere. Transmutation of minor actinides has a further benefit. They contribute significantly to the heat load of the spent fuel. Thus, their transmutation increases the effective capacity of a repository by at least a factor of 10, meaning the need for fewer repositories.

Radiotoxicity of spent nuclear fuel as a function of time

The separation of the elements of interest from nuclear waste (partitioning) and transmutation can be achieved in all of the foreseen fast reactors and in a dedicated system, such as the accelerator driven system (ADS). The latter device yields improved safety. It is a nuclear reactor operating in a sub critical condition, with additional neutrons required to maintain the reactor core criticality being provided by a particle accelerator coupled to the reactor. When the proton accelerator is switched off, the reactor essentially switches off.

The effort required to enable the deployment of such minor actinide management systems is significant. Advanced fuels need to be fabricated and their safety qualified. Fuels could be traditional oxide, metal, carbide or nitride chemical form. Table 1 compares some of their characteristics. Both oxide and metallic fuels operate at about 80% of their melting point. In contrast, carbide and nitride fuels operate at about 40% of their melting point. Despite their favourable margin to melt, neither carbide nor nitride have been deployed on a large scale, mainly due to difficulty in fabrication and a less well understood irradiation performance.

Table 1: Properties of metallic, oxide, nitride and carbide fuels

Fuel	Metal	Oxide	Nitride	Carbide
Melting Point (K)	1350	3000	3035	2575
Centreline temperature (K)	1050	2350	1000	1000

 

The cost of Germany’s nuclear phase out

Climate change is considered one of the greatest threats we currently face. Therefore, reducing CO2 emissions is considered a priority in most countries (at least on paper). To achieve this goal renewable energy must be a key component in the low-carbon energy mix. Where many would disagree is the role that Nuclear energy should play in this aspect.

Many have argue that nuclear energy can give us the time we need to phase out FOSSIL FUELS and Germany might become the best example on why nuclear energy is really needed. Germany will have to replace 23% of its energy generated by nuclear with something else. It is predicted that renewable energy will account for approximately 13% of the energy lost and the other 10% will come from gas, energy imports and a reduction in energy consumption. This increase of 13% of renewable energy could mean a 5-20% increase on the energy bills for each household. Germany will also have to invest 9.7 billion euros just to modify the grid to distribute the energy produced by renewables. No data is currently available on the direct cost of increasing the output of renewables by 13%.  Additionally, during winter when renewables are even less efficient but the energy demand is even higher the government will have to rely even more on coal, gas and imports from neighbouring countries.

Apart from this economic cost and with the current plan Germany will have little or no reduction on CO2 emissions since all massive investment on renewables will only go to replace part of the energy produced by nuclear power, a low-carbon emission energy source. Recently, the International Energy Agency have shown that the energy related CO2 emission reached a new high in 2010 with 30.6 Gt (billion tones). This value is considerably close to the emissions that were expected until 2020 of 32 Gt. Germany is about to show the necessity of having nuclear energy to drastically reduce CO2 emissions.

To put renewable energy a bit more into perspective, the installed wind power capacity in Germany in 2009 was of 25,777 MW but only produced 7% of the overall energy. On the other hand, nuclear had a capacity of 21,507 MW but produced almost 20% of the energy in the country.

(UPDATE: 26/09/2011)

New figures have appeared on the economic cost of Germany’s nuclear phase out. According to KfW Bankegruppe, Germany will have to invest around €25 billion per year (between €239-262 billion until 2020) if it wants to reach its target of 40% reduction on greenhouse gas emissions, including doubling the use of renewable energy and reducing energy consumption by 20%. These numbers include around €10 billion on 10 GWe produced from fossil fuels, €144 billion for renewable energy and €29 billion on 360 kilometres of high-voltage power lines.

On the human cost, EON has announced up to 11,000 job cuts due to heavy losses and the change in Germany’s policy.

Safety above all

Despite the valiant move from Germany, I seriously doubt other countries such as the UK, France, China, India, Sweden and the US will give up their nuclear power or stop building new plants any time soon. Therefore, safety of this technology must remain a paramount point in any future development in this area.

Currently, the Light Water Reactors (LWR) enclose their fuel in a metallic material (zirconium alloy) called cladding. One of the consequences of using this alloy is that during off-normal conditions the temperature of the reactor can go above 1200°C. Temperature at wich it starts to react with water vapour resulting in further release of heat (exothermic reaction) and hydrogen. This release of hydrogen was the origin of the explosions observed in Fukushima. As the temperature keeps increasing the uranium based fuel melts and produce the feared core meltdown.

New designs have addressed this problem in different ways. The one described in this post will be of the High Temperature Reactor (HTR). Instead of having large amounts of material encapsulated in long metallic tubes as happens in current LWR, the original creators of the HTR decided to put in practice the very well-known phrase “divide and conquer”. The fuel in this reactor is made of tiny spherical uranium particles, 0.5 mm in diameter, coated with 4 layers of ceramics (pyrolytic carbon and silicon carbide). The final size of this single unit fuel…. 1 mm in diameter. This type of fuel is known as TRISO (tristructural isotropic) coated fuel particle and it was originally created in the late 1960s in the UK during the Dragon project (not in Germany despite some erroneous believes). Hundreds of thousands of this TRISO particles are then mixed with graphite to form fuel compacts in spherical (pebble bed reactor) or cylindrical shape (prismatic core). The size of a fuel pebble is slightly bigger than a baseball ball. This type of fuel has several advantages over current designs. First, the coatings covering the uranium kernel keep all fission products inside this fuel, effectively creating a miniature fission product containment vessel. Imagine, all that steel and concrete replaced by layers of only a fraction of a millimeter. Second, is the stability of the material. Under off-normal conditions the temperature of the reactor can reach a maximum of 1600°C. This temperature, although very high for metallic materials, is considerably lower than the temperature necessary to melt/evaporate carbon or silicon carbide (>2100°C), posing no threat to the safety of the fuel. Other features are included in the reactors that in theory makes it physically impossible to have a core meltdown as happened in Fukushima. Many could say that this idea looks good on paper but we could not know in practice. Well, in 2004 the Chinese, who have a prototype reactor called HTR-10, decided to carry out a test that would have been considered unthinkable in current reactors. They decided to remove all cooling system (like happened in Fukushima) and study the safety of the reactor. The result, no damage to the fuel or the reactor giving strong, realistic evidence of the safety of this technology.